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            <ol class="post-toc"><li class="post-toc-item post-toc-level-3"><a class="post-toc-link" href="#对象的原型是什么？"><span class="post-toc-number">1.</span> <span class="post-toc-text">对象的原型是什么？</span></a></li><li class="post-toc-item post-toc-level-3"><a class="post-toc-link" href="#用构造函数创建对象"><span class="post-toc-number">2.</span> <span class="post-toc-text">用构造函数创建对象</span></a></li></ol>
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2018-07-19
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            <h3 id="对象的原型是什么？"><a href="#对象的原型是什么？" class="headerlink" title="对象的原型是什么？"></a>对象的原型是什么？</h3><p>Object是引用类型，包括：Object 、Array 、Function 、Data等。<br><strong>JavaScript对每个创建的对象都会设置一个原型，指向它的原型对象。</strong></p>
<p>当我们用obj.xxx访问一个对象的属性时，JavaScript引擎先在当前（this）对象上查找该属性，如果没有找到，就到其原型对象上找(数组是Array.prototype,function是function。prototype)，如果还没有找到，就一直上溯到Object.prototype对象，最后，如果还没有找到，就只能返回undefined。</p>
<p>比如创建了一个Array对象：<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">var arr = [1,2,3]</span><br></pre></td></tr></table></figure></p>
<p>其原型链是：<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">arr ----&gt; Array.prototype ----&gt; Object.prototype ----&gt; null</span><br></pre></td></tr></table></figure></p>
<p>Array.prototype定义了indexOf()、shift()等方法，因此你可以在所有的Array对象上直接调用这些方法。甚至你都可以在Array.prototype自定义方法，但是没有必要最后不要这样做。</p>
<p>当我们创建一个函数时：<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">function foo()&#123;</span><br><span class="line">  return 0;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure></p>
<p>函数也是一个对象 ，他的原型链是：<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">foo ----&gt; Function.prototype ----&gt; Object.prototype ----&gt; null</span><br></pre></td></tr></table></figure></p>
<p>由于Function.prototype定义了apply()等方法，因此，所有函数都可以调用apply()方法。</p>
<h3 id="用构造函数创建对象"><a href="#用构造函数创建对象" class="headerlink" title="用构造函数创建对象"></a>用构造函数创建对象</h3><p>用函数来创建对象比直接用var = {…}创建对象要方面的多，它就是构造函数。<br>构造函数的首字母都应该大写<br>首先先创建一个构造函数：<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">function Student(name) &#123;</span><br><span class="line">    this.name = name;</span><br><span class="line">    this.hello = function () &#123;</span><br><span class="line">        alert(&apos;Hello, &apos; + this.name + &apos;!&apos;);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure></p>
<p>可以用关键字new来调用这个函数，并返回一个对象：<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">var xiaoming = new Student(&apos;小明&apos;);</span><br><span class="line">xiaoming.name; // &apos;小明&apos;</span><br><span class="line">xiaoming.hello(); // Hello, 小明!</span><br></pre></td></tr></table></figure></p>
<p>注意，如果不写new，这就是一个普通函数，它返回undefined。但是，如果写了new，它就变成了一个构造函数，它绑定的this指向新创建的对象，并默认返回this，也就是说，不需要在最后写return this;。</p>
<p>新创建的xiaoming的原型链是：<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">xiaoming ----&gt; Student.prototype ----&gt; Object.prototype ----&gt; null</span><br></pre></td></tr></table></figure></p>
<p>也就是说，xiaoming的原型指向函数Student的原型。如果你又创建了xiaohong、xiaojun，那么这些对象的原型与xiaoming是一样的：<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">xiaoming ↘</span><br><span class="line">xiaohong -→ Student.prototype ----&gt; Object.prototype ----&gt; null</span><br><span class="line">xiaojun  ↗</span><br></pre></td></tr></table></figure></p>
<p>用new Student()创建的对象还从原型上获得了一个constructor属性，它指向函数Student本身：<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">console.log(xiaoming.constructor === Student)//true</span><br></pre></td></tr></table></figure></p>
<p>我们可以用instanceof来检测对象类型；<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">console.log(xiaohong instanceof Object);//true</span><br><span class="line">console.log(xiaohong instanceof Student);//true</span><br><span class="line">console.log(xiaoming instanceof Object);//true</span><br><span class="line">console.log(xiaoming instanceof Student);//true</span><br></pre></td></tr></table></figure></p>
<p>这个例子中所创建的对象即是Object的实例 也是Student的实例，因为所有的对象都继承自Object</p>
<p>参考：js高程第六章<br>参考：<a href="https://www.liaoxuefeng.com/wiki/001434446689867b27157e896e74d51a89c25cc8b43bdb3000/0014344997235247b53be560ab041a7b10360a567422a78000" target="_blank" rel="noopener">廖雪峰的博客</a> </p>

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